1. Field of the Invention
The present invention relates to a rotating prism Q-switch for use with CO.sub.2 TEA lasers, wherein the rotating prism Q-switch uses the principle of internal reflection at near the critical angle to produce an optical loss that switches between a substantial loss for angles less than the critical angle and near zero loss for angles equal and greater than the critical angle.
2. Description of the Prior Art
The CO.sub.2 laser has long been available and can be configured to produce a continuous or pulsed laser beam. It is capable of high average power output while, at the same time, maintaining a high degree of spectral purity and spatial coherence. An electric discharge is the most common means of excitation. Operating efficiency and output power are greatly increased by adding nitrogen and helium to the fill gas. Helium aids depopulation of the terminal laser level and N.sub.2 excites CO.sub.2 molecules by collisional energy transfer. To facilitate the discharge, the CW excited CO.sub.2 laser is operated at low pressure, on the order of 100 torr.
It is possible to store energy in the discharge medium by blocking the path of the laser beam within the resonator, thereby preventing laser oscillation. If the block is suddenly removed, the output from the laser occurs in the form of a short duration pulse with peak power two to three orders of magnitude larger than the average continuous-wave power. This mode of operation is called Q-switching. In a typical prior art device in which the gas is excited by CW discharge, Q-switching is accomplished by replacing one of the laser cavity mirrors with a rotating mirror. A laser pulse is produced every time the rotating mirror lines up with the opposite stationary mirror.
A more efficient method of producing high peak power pulses from the CO.sub.2 laser is the use of a pulsed high voltage discharge in a gas medium at much higher pressure. This is called the CO.sub.2 TEA laser (transversely excited atmospheric) in which the gas pressure is near one atmosphere and the discharge is very fast and transverse to the beam axis. By operating at higher pressure, the density of excited CO.sub.2 molecules is increased, thereby proportionally increasing the peak power output. The added difficulty of creating the discharge in the higher pressure gas is offset by the reduced path length of the transverse discharge. The high peak power of the CO.sub.2 TEA laser is not accomplished by a Q-switch, but is a result of the fast discharge which causes the gain to build up faster than the laser pulse. This method is called "gain switching".
The fast discharge method is undesirable for many laser applications because sufficient nitrogen excitation remains after the initial laser pulse to sustain laser oscillation at a power level 1/10 to 1/4 of the peak. The output energy after the main pulse, commonly referred to as the "tail", typically contains more than half the energy and lasts several microseconds. In laser range finder applications, the tail is back scattered into the receiver thus "blinding" the receiver for the few microseconds that the tail exists, an unacceptable situation.
The tail can be eliminated by the addition of a Q-switch, wherein the Q-switch is on for the main pulse, and then turned off to prevent the tail. The Q-switch also can be operated to increase the peak output power by delaying the initial switch opening so that the laser pulse occurs near peak gain. For the gain switched laser, the laser pulse build up may occur well before peak gain thereby increasing the tail energy.
Since the CO.sub.2 TEA laser has an excited state lifetime of only a few microseconds, a timing accuracy of a few hundred nanoseconds is required for the time delay between the gas discharge and Q-switch opening. Heretofore, only the electro-optic Q-switch was capable of providing this degree of timing accuracy. However, the electro-optic Q-switch has serious disadvantages when used with CO.sub.2 TEA lasers due to its cost, complexity, fragility, and susceptibility to laser damage.
An arrangement adapting a rotating prism Q-switch for use in a CO.sub.2 TEA laser is disclosed in copending U.S. patent application Ser. No. 637,097, now U.S. Pat. No. 4,660,204, entitled "A CO.sub.2 TEA Laser Utilizing on Intra-Cavity Prism Q-Switch" by Donald R. Dewhirst and Robert L. Duvall III filed Aug. 2, 1984. The copending application discribes a rotating prism Q-switch in which the prism is interposed between the laser resonator mirrors and is arranged such that the rotation of the prism sweeps the resonator mirrors through alignment with one another once each revolution of the prism. The copending application also describes an opto-electronic timing device, used as well by the present invention, in which the necessary timing accuracy is provided to synchronize the gas discharge with the rotating prism. Whereas the Q-switch of the copending application operates by altering the resonator alignment, the Q-switch of the present invention maintains good resonator alignment, but switches the optical transmission through the prism.